When a fuel such as heavy oil, residual oil or coal is burned, sulfur contained in the fuel will be burned to form sulfur dioxide (SO2). Further, this sulfur dioxide will be partially oxidized to form sulfur trioxide (SO3), which will be reacted with moisture in the flue gas to form sulfuric acid (H2SO4). Accordingly, on an apparatus or piping disposed between a combustion furnace of a boiler and a chimney, a mixture containing unburned ash and an acidic component derived from the fuel, such as sulfuric acid, is likely to be deposited. Especially when the temperature of the flue gas is lowered to a level of not higher than the dew point of the sulfuric acid, such deposition tends to be remarkable, and cleaning of the apparatus or piping is essential to maintain the stable operation.
Heretofore, it has been common to employ a cleaning method to wash off the acidic deposit on equipments by using industrial water. As a method for effectively removing the acidic deposit without exposing the instruments to a corrosive environment, a method has been practically employed which utilizes a neutralization reaction by means of sodium hydrogencarbonate as shown in Patent Document 1.
In the cleaning, it becomes important to ascertain the finish state. As in the cleaning of the interior of an apparatus or the interior of slender piping, it is not necessarily possible to carry out the cleaning while the cleaning state of the portion to be cleaned, is observed. Therefore, in the case of washing off the acidic deposit with water or in the case of washing by means of sodium hydrogencarbonate, it has been common to ascertain the cleaning state by observing the change in the value of the hydrogen ion concentration (pH) of the cleaning fluid. For example, in a case where cleaning is carried out by circulating an aqueous solution having an excess amount of sodium hydrogencarbonate dissolved in water, neutralization of the acidic deposit will advance as the cleaning progresses, and the pH value of the cleaning fluid will increase gradually, and it has been considered possible to judge that the cleaning has been done when the cleaning fluid has become to show the pH value of the aqueous sodium hydrogencarbonate solution itself.
Patent Document 1: JP-A-2001-348689